In recent years there has been growing interest in the use of polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based devices are achieving efficiencies up to 4-5%. This is appreciably lower than the efficiencies attainable by inorganic devices, which are typically up to 25%.
The class of polymers currently achieving the highest efficiencies in OPV devices are the poly(3-alkyl-thiophenes). The most commonly used example is poly(3-hexyl-thiophene), P3HT, due to its broad availability and good absorption characteristics. P3HT absorbs strongly over the 480-650 nm range, with a peak maximum absorption at 560 nm. This means a significant portion of the light emitted by the sun is not being absorbed.
In order to improve the efficiency of OPV devices, polymers are required that absorb more light from the longer wavelength region (650-800 nm). For this purpose, polymers are desired which have a low band gap, preferably less than 1.9 eV, whereas for example P3HT has a band gap of ˜2.0 eV.
Low band gaps are attained in polyaromatic conjugated polymers with a high quinoidal contribution. Poly(thiophene), for example, can exist in both the aromatic and quinoidal state as shown below:

A quinoidal structure reduces the torsion between adjacent rings, which results in a more planar polymer backbone leading to an extension of the effective conjugation length. It is generally observed in conjugated polymers that an increase in the conjugation length results in a decrease of the bandgap.
The quinoidal state can be stabilised by fusing an aromatic ring to the thiophene backbone. The fused ring is only fully aromatic when the backbone is in the quinoidal state. This means there is a strong desire for the polymer to be in the quinoidal state. Previous work (see J. Roncali, Chem. Rev., 1997, 97, 173 and references cited therein) has demonstrated the use of a benzo or naptho fused thiophene, as shown below, to reduce the bandgap:

However, in these cases the fused six-member ring can cause steric strain by interaction with the neighbouring thiophene monomers. This can result in undesirable twists in the backbone, and a concurrent reduction in effective conjugation. By fusing a five-member ring steric interactions are reduced. One related example as shown below demonstrates the use of a fused thiophene ring:

However, this type of structure is synthetically very complex (M. Pomerantz, X. Gu and S. X. Zhang, Macromolecules, 2001, 34 (6), 1817).
It is an aim of the present invention to provide new materials for use as semiconductors or charge transport materials, which have the desired properties as described above, especially a low band gap, high charge mobility, good processibility and oxidative stability, and furthermore are easy to synthesize. Another aim of the invention is to provide new semiconductor and charge transport components, and new and improved electrooptical, electronic and luminescent devices comprising these components. Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing mono-, oligo- and polymers as claimed in the present invention, which comprise a 2-substituted thieno[3,4-d]thiazole-6,4-diyl unit of the following structure:

The materials according to the present invention comprise a five-membered thiazole ring that is fused to the thiophene. This does not only increase the quinoidal contribution compared to thiophene, but, being more electron deficient, it also helps to improve the stability by withdrawing electron density as shown below:

I. T. Kim, S. W. Lee and J. Y. Lee, Polymer Preprints, 2003, 44(1), 1163 disclose the homopolymer poly(2-nonyl)thieno[3,4-d]thiazole:

The reference gives details on the synthesis and characterisation of the homopolymers for use as low band gap conducting polymers. The homopolymer absorbs in the 650-800 nm region, with a peak maximum absorption at 725 nm. The band gap for this polymer, calculated using the bandedge from the UV-vis-NIR and cyclic voltammetry, was found to be 1.3 eV and 1.26 eV respectively. However, the polymer is prepared electrochemically, which is not conducive to large scale preparations. Moreover, copolymers are not described in this reference.